Open Path Fourier Transform Infrared (OP-FTIR) Spectroscopy is a remote sensing air monitoring technique that was first used in the 1960's for the purpose of measuring the atmospheres of earth and other planets. It is based on the traditional line of FTIR spectrometers, which started with Michelson's invention of interferometry in 1891. FTIR spectroscopes have been used for the evaluation of solid, liquid, and gas analytes since that time. Bench-top gas FTIR spectrometers are the most similar spectrometers to OP-FTIR spectrometers, as both are designed to measure the infrared spectra of gases in a sample space by way of interferometry. The bench-top FTIR is used to analyze gas analytes in an enclosed compartment or cell. The cell can be purged with nitrogen or even evacuated to obtain a background spectrum, and then filled with the analyte for analytical spectrum measurement.
The concept of a background spectrum is central to spectroscopy. In the simplest terms, Beer's law relates the amount of a substance present in an analytical sample to the amount of light absorbed by the analytical beam. The substance absorbs light, so more of the substance means less light passes through the sample space. To understand what is in an analytical spectrum, the light that is absorbed or not transmitted is measured. To make this measurement, it is necessary to know how much light would be there without the analyte present. The background spectrum (I0) ideally represents what the spectrum would look like without the analyte. The issue is trivial with a bench-top unit, since the cell can be filled with nitrogen, which has almost no features in the mid-infrared region of the spectrum.
The OP-FTIR spectrometer is radically different, since its sampling volume is the open atmosphere. The open atmosphere cannot be put into a cell, and the necessary innovation of OP-FTIR was the use of the sun or another distant infrared source enabling for the first time, the powerful FTIR technique to be used as a remote sensing technology. This same remoteness of the environment being evaluated prevents the sample space from being filled with a vacuum, nitrogen, or any other non-infrared absorbing gas, and as a result, the background problem is created. Under such uncontrolled conditions, it is not possible to measure what would be in an open path without the analyte present.
The limitations in obtaining the ideal background spectrum have not prevented OP-FTIR from being useful for monitoring air pollutants in a variety of terrestrial environmental and workplace situations. These include the monitoring of hazardous waste sites, waste lagoons from confined animal feeding operations, smog analysis, volcanic emissions, and chemical plants. The transportability of the measurement system has allowed it to be flexibly used for mobile source monitoring, stack monitoring, workplace monitoring, and urban air pollution monitoring. The most common gaseous air pollutants regulated by the U.S. Environmental Protection Agency (US-EPA), termed “criteria” pollutants, are lead, particulate matter, SO2, CO, ozone (O3), NO2, and NO. All of these gases can be detected by OP-FTIR, and some work has been done on detecting particulate matter, a non-gaseous criteria pollutant, as well. These gases typically are monitored by a suite of instruments, using one for O3, one for NO and NO2, one for SO2, and a fourth separate instrument for CO.
The 1990 Clean Air Act Amendments expanded the US-EPA list of air pollutants by adding 189 Hazardous Air Pollutants (HAPs) to the original criterion air pollutants as potentially dangerous compounds that should be monitored. OP-FTIR spectroscopy has proven particularly useful for the identification and quantification of a number of these compounds due to their unique infrared absorption features. Many of these compounds are larger organic molecules and would require capture in a bag, charcoal tube or canister, and transport to a lab for analysis by gas chromatography or mass spectroscopy. These methods are accurate but very costly for each sample and usually not able to obtain time resolution better than an hour. OP-FTIR can theoretically simultaneously monitor for many of these compounds, along with the criterion pollutants, over distances on the scale of hundreds of meters, and in real time.
Fence line and area sources have been the focus of most applications of this technology due to the nature of a beam average concentration. Since the sample space is spread along the beam-path, the concentration calculated represents a path integral measurement. The path integral measurement, which represents the total number of molecules in the path, can be converted into a path average measurement by dividing by the path-length. This conversion is beneficial when trying to calculate a flux from an area source with irregular source strength and/or changing wind direction. Another benefit is the speed of data acquisition; OP-FTIR offers much better time resolved data than other techniques, such as evacuated SUMMA™ canisters analyzed by gas chromatography-mass spectrometry (GC-MS), which collect samples typically integrated over several hours. With OP-FTIR, it is possible to obtain valid spectra every few minutes, and to quantify these spectra for any combination of the aforementioned gases. In theory, any gaseous compound having a dipole moment can be detected in this manner. The quantification of several species simultaneously is typically done using Beer's Law and classical least squares techniques. The resulting output data are in units of parts per million-meters (ppm-m) and represent the fence line or ambient concentration measurement integrated over the entire beam path. Both stable and unstable compounds can be measured with OP-FTIR in this way, since no sample handling or storage is involved. This benefit makes OP-FTIR a useful tool for monitoring changes in reactive atmospheric gases and for quantifying reaction products that play a role in urban air pollution.
Cost also is an important consideration when selecting a sampling method; the initial cost of an OP-FTIR is considerable (˜$100,000 when done by Cerex Environmental Services in 2007) but has been decreasing in recent years. Yet, when compared to the cost of several monitoring devices that a single OP-FTIR could be replacing, and the lack of sample transport and lab analysis costs for the HAPs, the total operational cost of OP-FTIR can be competitive with conventional sampling for many applications.
The main advantages of OP-FTIR can be summarized as follows:
1. Flexibility for simultaneous monitoring of multiple gases;
2. Fast sample collection & analysis;
3. Average over path less susceptible than a point sample to a small spike in space;
4. Long-term cost is comparable to or cheaper than other methods; and
5. Measurement of reactive species that cannot be captured and analyzed later.
Despite these advantages, the full promise of OP-FTIR has not yet been fulfilled. Although OP-FTIR has many advantages as described above, its optimal use is constrained by two key problems. First, the instrument is characterized by an inherent inability to determine the physical location or magnitude of the maximum point concentration of a pollutant along the beam path. The path average ppm-meters quantity is helpful in determining fluxes for area sources, but unable to determine a peak concentration in space. This “spatial resolution problem” has been the subject of considerable research and can be solved by using multiple beam paths.
Second, the OP-FTIR requires a true background spectrum to calculate concentration using the Beer Lambert law. As mentioned above, this “background problem” stems partially from the inability to control environmental factors (mainly temperature and relative humidity (RH)) in an open sampling volume. Additional factors that contribute to the background problem are slight changes in instrumental parameters such as resolution and spectral shift.
The spatial resolution problem exists because the data collected by open path instruments are in the form of an integral of the concentration along the entire beam path, expressed in ppm-meters, which means that a one meter wide plume of 100 ppm concentration at any point along the path-length would create the same instrument response as a 100 meter wide plume of one ppm. Effectively, the only concentration value that can be obtained from a single path integrated measurement is the average over the entire beam path. Since health effects depend greatly on the air concentration at a particular location, for exposure assessment in workplaces or near source and fence line monitoring applications, this lack of spatially resolved concentration information is unacceptable. However, by combining data from multiple beam paths of different lengths, it becomes possible to describe the spatial concentration distribution over a desired sampling space.
Ambient monitoring for urban air pollution applications should not be affected by spatial resolution, since the pollutants typically are well mixed on a scale of hundreds of meters. Therefore, the lack of spatial resolution is of little consequence for this situation. However, urban air monitoring often requires monitoring of trace compounds with very low detection limit goals (i.e., the monitoring instrumentation must have great sensitivity), and consequently, can be affected by interference and fluctuations in light transmission due to variations in the normal constituents of the atmosphere in the beam path. The I0 or background spectrum determination has been a serious problem and the subject of substantial research. Water vapor, CO2, particles, and slight variations of instrument performance can all affect the data and make a particular analytical spectrum inappropriate for a given background spectrum. These issues can severely degrade detection limits and lead to an apparent increase in instrument noise.
The OP-FTIR “background problem” comprises many parameters. A single beam spectrum is affected by numerous environmental and instrumental factors. The method is based upon the differences between I0 and IA caused by the target gas. All of the other factors can also change between I0 and IA. Essentially, what is needed to solve this problem is a spectrum other than the analytical spectrum that is identical to the analytical spectrum, except for the lack of target gases.
Accordingly, it would be desirable to address the background problem through the use of single-beam spectra for determining temperature and concentration of water and other gases for each analytical spectrum. In OP-FTIR spectroscopy, the water vapor interference and its temperature dependence are a subset of this background problem that can be of particular difficulty. Water vapor is always present in the atmosphere, so it is a background issue. The amount of water vapor present is also usually changing at a rate that makes it an interfering gas as well, and is at the percent level, which is orders of magnitude more than any pollutant. Thus, a small error in the calculation of water vapor concentration can result in a large error in the concentration of an analyte. Typically, interfering gases are included in the quantification method, and their concentrations are calculated along with the target gases. However, since water vapor is such a dominant absorber, it is best to use a background spectrum having a water content that is as close to the expected level as possible. This approach does not preclude the need to use water in the quantification method as an interfering species.
Water vapor has absorption features that interfere with potential target gas absorption in most of the usable bandwidth of an FTIR. Bench-top spectrometers have had to deal with this problem since their inception. Even the water vapor contained in a few inches of ambient air within a sample space can cause limitations. This problem is magnified for OP-FTIR instruments because their path-lengths are longer. Water vapor content is often the limiting factor for the maximum path-length for a given part of the spectrum. The band from 1200 cm−1 to 1800 cm−1 contains important features for many gases, but is unusable in OP-FTIR due to the water vapor absorption. Deuterated (heavy) water (HDO) molecules that include a deuterium hydrogen isotope as one of the two hydrogen atoms represent only 1/8,000 of water molecules in the atmosphere. Even so, the HDO features are clearly seen in OP-FTIR single beams due to the magnitude of water vapor in a typical path. FIGS. 1A and 1B show that water vapor absorbs in the entire working range from 400 cm−1 to 4,000 cm−1.
In an exemplary infrared absorption spectrum graph 100 (and a more detailed graph 100a shown respectively in and FIGS. 1A and 1B, hundreds of individual absorption lines with great variation in their magnitude are displayed. As will be evident in graph 100a, in FIG. 1B, the absorbance of water vapor in air is never actually zero. The tails of all of the lines in the mid-infrared spectrum and beyond add to form a continuum spectrum. Thus, even between lines, there is some absorption by water vapor.
FIG. 2 is an exemplary graph 110 illustrating the transmittance spectrum of water vapor in air, which shows that the plot never quite reaches 100% transmission (transmittance of 1) between lines, even at the relatively short and dry path of 64 meters and 10,000 ppm H2O. This lack of points with 100% transmission leads to difficulty in some methods of creating a background from the field spectrum. These methods for creating backgrounds depend on interpolating between absorption-free points on the spectrum without water absorption.
Water molecules absorb infrared radiation by entering higher rotational and vibrational energy states. This phenomenon creates the complex infrared spectral features. At any specific ambient temperature, the ratios of water molecules in the different energy states reach a unique equilibrium. Because the spectral lines are a function of the initial energy state distribution, the infrared spectrum of water vapor is considerably temperature dependent. The differences in the spectrum that result from temperature variation at a given concentration can exceed the instrument noise and the absorption of target gases. Therefore, it is crucial to have a water reference and/or water containing background with the correct temperature as well as water concentration for each analytical spectrum. An acceptable background spectrum must have these two parameters correctly represented, as well as any variable instrument parameters, to properly account for the water vapor that is present.
The answer to this problem has increasingly been the use of synthetic spectra created with the high-resolution transmission molecular absorption database (HITRAN) or another spectral database. HITRAN is a compilation of spectroscopic parameters (i.e., a database) that a variety of computer codes use to predict and simulate the transmission and emission of light in the atmosphere. The creation of this database is a long-running project started by the Air Force Cambridge Research Laboratories (AFCRL) in the late 1960's in response to the need for detailed knowledge of the infrared properties of the atmosphere. It is unreasonable to try to create a comprehensive library by collecting water vapor spectra over the entire range of concentrations, path-lengths, and temperatures that would be needed in the field. Others have done considerable work with this idea and have created software such as “NONLIN” and “MALT,” among others, that synthetically generates reference spectra for each analytical spectrum for several gases including water and CO2. The ambient temperature is an important input to all of these models that the user must measure carefully for each spectrum. Temperature, however, can vary over distances of several hundreds of meters, especially over a water surface or other differing surfaces. Many applications of OP-FTIR have beam-paths of this length over non-homogeneous surfaces. Errors in the temperature input to these models results in errors in the size and shape of spectral features in the synthetic spectra. However, it is clear that developing an approach for accurately determining the true average temperature of the molecules along the entire beam would result in more accurate synthetic spectra and better quantification of the gases in the beam, which can be invaluable in many applications of OP-FTIR.